from tkinter import *

def resize(ev=None):  # IDK what ev=None does.
    label.config(font="Helvetica -%d bold" % scale.get())

top = Tk()
top.geometry("250x150")

label = Label(top, text="Hello World!", font="Helevetica -12 bold")
label.pack(fill=Y, expand=1)

scale = Scale(top, from_=10, to=40, orient=HORIZONTAL, command=resize)
scale.set(12)
scale.pack(fill=X, expand=1)

quit = Button(
    top, text="QUIT", command=top.quit,
    activeforeground="white", activebackground="red"
    )
quit.pack()
mainloop()

